In a typical TDMA (time-division multiple-access) cellular communications system, a large geographic region is subdivided into cells and, for each cell, at least one base station is given the responsibility of communicating with mobile stations located within that cell. In the so-called "reverse link" direction of communication, mobile stations transmit data in assigned time slots using assigned carrier frequencies. While adjacent cells always use distinct carrier frequencies, cells which are separated by at least one other cell may use the same carrier frequency.
If a digital modulation technique known as quaternary phase-shift keying (QPSK) is used, then the data to be transmitted by a mobile station is grouped into pairs of bits, each pair identifying one of four possible symbols. The modulated radio-frequency signal transmitted by the mobile station thus has a "transmitted symbol rate" which is one-half the data bit rate; the time interval between two transmitted symbols, known as the "transmitted symbol period", is expressed as T.sub.s.
Several mobile stations can share a carrier frequency by virtue of being assigned distinct time slots in which they can transmit their respective modulated radio-frequency signals. The TIA/EIA standard known as IS-54, and incorporated by reference herein, allows a maximum of six mobile stations to share a single carrier frequency by subdividing each recurring 240 millisecond time interval, referred to as a TDMA frame, which is further subdivided into six individually assigned time slots. With reference to FIG. 1A, a time slot 2 in normal burst format in accordance with TIA/EIA standard IS-54 comprises 324 bits and is subdivided into several fields, including a SYNC field 4 (of length 28 bits), three DATA fields 6,8,10 (of length 16, 122 and 122 bits, respectively) and a coded digital verification color code (CDVCC) field 12 (of length 12 bits).
It is noted that when QPSK is employed, the 324 bits are not transmitted individually, but rather are grouped by pairs into 162 symbols for transmission. Thus, when subsequent pairs of bits forming the 28-bit SYNC field 4 are assembled into symbols (phase vectors), the SYNC bit pattern may be represented by a fourteen-element complex-valued SYNC vector. Similarly, by grouping the 12 bits in the CDVCC field 12 into pairs, each pair forming a symbol, the CDVCC bit pattern may be represented by a six-element complex-valued CDVCC vector.
The SYNC field 4 is used for timing and carrier recovery at the base station. Although the SYNC field 4 is required to be distinct for each of up to six time slots multiplexed onto recurring frames transmitted at a given carrier frequency, the same set of six SYNC bit patterns are also associated with the six available time slots corresponding to a different carrier frequency. On the other hand, the CDVCC field 12 is uniquely associated with each cell and is used by a mobile station to identify the cell.
During handoff and other scenarios, it is of paramount importance to ensure that a base station can perform rapid and reliable validation of the CDVCC vector. This is true for any cellular system and in particular for one which is compliant with IS-54. In theory, rapid validation of the CDVCC vector can be achieved by a base station detecting the SYNC vector (which can be chosen to possess strong correlation properties), establishing a relatively accurate clock signal, detecting the CDVCC vector and comparing it to the CDVCC vector uniquely identifying the base station.
However, multipath or other dispersive effects of the radio-frequency channel may cause the SYNC vector to be detected with some delay. For instance, FIG. 1B illustrates the correlation function of a detected SYNC vector with the known SYNC vector under ideal conditions (top) and under multipath conditions (bottom). Specifically, the detected SYNC vector may be found to have a high correlation with the known SYNC vector when the detected SYNC vector is delayed from its true position by up to one full transmitted symbol period T.sub.s (or more, in some cases). If multipath effects vanish by the time the CDVCC vector is expected to be received, i.e., after the expiry of 67 transmitted symbol periods as shown in FIG. 1A, then the true position of the CDVCC vector may in fact precede, by up to T.sub.s seconds, the position at which it is thought to be, based on the position of the detected SYNC vector.
To solve this problem, it might appear plausible to attempt detection and verification of the CDVCC vector using an correlation-based method similar to that employed for detection of the SYNC vector. However, it is noted that the CDVCC vector is uniquely associated with a given cell and therefore cannot be made to possess good auto-correlative properties for all cells, which renders this rudimentary technique unreliable for validation of the CDVCC vector.
One known approach is for the receiver to detect the SYNC vector and then to adaptively track channel variations for each and every one of the subsequent symbols in order to arrive at a precise estimate of where the CDVCC vector actually begins. However, this method is prohibitively complex, as it requires very fast tracking of the channel using computationally intensive adaptive algorithms. Furthermore, the channel tracking suffers from low accuracy in the presence of demodulated symbol errors.